Gas turbine engines employ a set of rotating turbine blades to compress air leading to a combustion chamber into which fuel is injected and ignited. The hot gases from combustion turn a downstream set of blades from which energy is extracted and which are also typically connected to a common shaft to turn the compressor blades.
Fuel is delivered to metering orifices in the combustion chamber under pressure through a fuel line. When the engine is to be shut down, the fuel pump supplying fuel is turned off, a fuel line valve closed, and the fuel line purged with hot gases from the turbine compressors. The purging prevents fuel from "coking" at the metering orifice. In coking, the volatile components of the fuel are driven off leaving only a tarry deposit.
In order to permit purging of the fuel line, a purging air line must join with the fuel line. Because it is important that fuel not go down the air purge line and that hot gases not travel up the fuel line to its source, typically check valves are installed in each of these lines. Such check valves may employ a spring-loaded valve member that closes when the pressure drop across the valve member drops to zero or reverses. In this application, such valves may be unreliable, sticking in the open position as a result of debris or the like. Further, there is inevitably some backwash through the valves as they close, letting hot gases into the fuel line or fuel into the hot gas purge line with the result that coking fuel coats the check valve mechanism further reducing its reliability.
The fuel cut-off valve may be controlled by a pneumatic actuator operating a valve in line with the check valve of the fuel line. Such a valve is also subject to coking and may introduce substantial pressure drops in the fuel flow. The fuel cut-off valve introduces a potential failure point to the turbine where, if pressure is lost to the pneumatic actuator, the turbine will cease operating.
What is needed is a mechanism for air purging and fuel control, preventing backwash, more resistant to coking, yet more reliable than provided by conventional cutoff valve check combinations.